To be clear (no pun intended), Clear lets you jump to the start of the PreCheck line too (they "stack").
This summer in Denver, for example, the pre-check line took about 30-45 minutes, wrapping around several parts of the airport.
The Clear line (which I didn't have access to) jumped to the end of head of that queue, cutting off like 20-30 min.
Many travel credit cards will pay for PreCheck anyway, but only a few will pay for Clear.
But having both will be the fastest. Often it won't matter, but when it does, it's nice.
As a consequence, many people have signed up so…now Clear sometimes has a big line.
The TSA-pre line wraps around but goes super fast, almost walking pace. The clear line barely moves.
Clear has become a gimmick that makes you believe you are going through faster because the line is shorter.
I was in the Precheck line without Clear, and the Precheck + Clear people moved way faster to the head of the line I was in.
(To be clear, there are two separate Clear lines in many airports, one for people with Precheck and one for people without it. So four lines total, normal/precheck/normal+clear/precheck+clear).
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But regardless, sometimes one of the lines will be shorter than others. Sometimes regular is faster than Precheck too (but you'll have to take off shoes and take laptops out, etc.). Some entrances will also have shorter lines than others.
But having both PreCheck + Clear means you can choose from any line you want. If the PreCheck line without Clear is faster, well then, just go in that and skip Clear this time.
I know it is counter-intuitive because the clear line is so much smaller, but every single time it barely moves. Check yourself next time.
Anecdotal only. I fly fairly regularly.
Clear lets you bypass the pre-check line and go straight to the head of the line. People will pay extra for that. And they'll gladly give up their fingerprints and retina prints. My wife certainly did, and she's a lawyer.
i.e. Clear lets you jump straight to the security part. Many airports will have a dedicated Precheck+Clear lane, which would typically be faster than either Precheck or Clear on their own.
The middle security checkpoint (near gate 12) also has a Clear entrance, and that has always been empty every time I've been there. The Precheck line might be empty, or it might be longer than the regular line -- I've seen both types of situations, on multiple occasions.
The West checkpoint (near gate 27 and above) is one I'm not as familiar with. I don't know if they have a Clear line or a Precheck line. We almost never go through there.
The middle checkpoint has Clear, Precheck, and regular. There's no fourth line -- if you're in the Clear line, they just take you to the head of whichever other line you're going through.